Many applications require individual populations of cells to be separated from heterogenous samples, for example the isolation of fetal cells from maternal blood, blood cell fractionation, or the separation of circulating tumour cells from the peripheral blood of cancer patients.
Chemical separation methods based on affinity capture of cell surface molecules are very effective at isolating cells with known chemical markers, but may alter the properties of the cells. Where the components of the sample need to be preserved in their original state, chemical methods are generally undesirable. Affinity capture methods are generally not possible where no unique biomarker is known.
The separation of cells based on their physical differences is important in many areas of medical research and clinical practice. Previous technologies for physical separation include dielectrophoresis (DEP), hydrodynamic chromatography, which separate cells based on size alone, and filtration, which separate cells based on size and rigidity. Separation based on size and rigidity is generally considered to be more useful since size alone is often insufficient to distinguish different cell types. These physical sorting methods sort cell populations on the basis of physical properties such as conductivity, size, deformability or combinations thereof. A recurring limitation in the filtration of cells is clogging, or the build up of particles within the filter microstructure. Clogging alters the hydrodynamic resistance of the filter, causing loss of specificity, yield, and throughput. Additionally, constant contact between the cell membrane and the filter wall can increase the incidence of cells adsorbing on to the filter wall and, in turn, prevent the recovery of cells after separation.
In applications where the target cell population is present at a very low concentration, cell sorting methods that allow for efficient separation and recovery of cells while avoiding clogging of filters, exposure cells to excessive pressures, with very high efficiency and purity are particularly desirable.
Previous micro-scale ratchet mechanisms utilize a periodic structure having local asymmetry and local excitation to modify the motion of individual particles against the viscous drag of the particle's carrier fluid (Astumian, R. D., Thermodynamics and kinetics of a Brownian motor. Science, 1997. 276(5314): p. 917-922; Julicher, F., A. Ajdari, and J. Prost, Modeling molecular motors. Reviews of Modern Physics, 1997. 69(4): p. 1269-1281). Typically, micro-scale ratchet mechanisms exploit asymmetries of a flow on the basis of electrical potential (Bader, J. S., et al., DNA transport by a micromachined Brownian ratchet device. PNAS, 1999. 96(23): p. 13165-13169), dielectrophoresis (Gorre-Talini, L., J. P. Spatz, and P. Silberzan, Dielectrophoretic ratchets. Chaos, 1998. 8(3): p. 650-656; Rousselet, J., et al., Directional Motion of Brownian Particles Induced by a Periodic Asymmetric Potential. Nature, 1994. 370(6489): p. 446-448), optical traps (Faucheux, L. P., et al., Optical Thermal Ratchet. Physical Review Letters, 1995. 74(9): p. 1504-1507), geometrical constraint imposed by obstacles (Loutherback, K., et al., Deterministic Microfluidic Ratchet. Physical Review Letters, 2009. 102(4): p. 045301; Matthias, S. and F. Muller, Asymmetric pores in a silicon membrane acting as massively parallel brownian ratchets. Nature, 2003. 424(6944): p. 53-57), and bacteria and cell motility (Galajda, P., et al., Funnel ratchets in biology at low Reynolds number: choanotaxis. Journal of Modern Optics, 2008. 55(19-20): p. 3413-3422; Hulme, S. E., et al., Using ratchets and sorters to fractionate motile cells of Escherichia coli by length. Lab on a Chip, 2008. 8(11): p. 1888-1895; Mahmud, G., et al., Directing cell motions on micropatterned ratchets. Nature Physics, 2009. 5(8): p. 606-612). The asymmetries in a flow correlate with particle size, and as such, are useful for size-based separation (Davis, J. A., et al., Deterministic hydrodynamics: Taking blood apart. Proc. Natl. Acad. Sci. U.S.A., 2006. 103(40): p. 14779-14784).
It is, therefore, desirable to provide improved apparatus and methods for particle separation.